The present invention relates to prosthetic systems for the replacement of joints or portions thereof. More particularly, the invention concerns an ankle prosthesis system that can be used in the extremities that have experienced bone loss or significant, irreparable bone trauma.
For treatment of various problems with the ankle such as degenerative arthritis and trauma of the ankle, total ankle replacement, or “arthroplasty” is rapidly becoming a common course of action. One method of providing relief to a patient is to replace the articulating surfaces of the ankle, i.e. the inferior articular surface of the tibia and the articular surface of the talus. The inferior articular surface of the tibia is replaced with a concave polymer bearing, and the articular surface of the talus is replaced with a convex metal bearing. The polymer used can be polyethylene, for example. In such replacement, pain relief, increased motion and anatomic reconstruction of the ankle joint are goals of the orthopaedic surgeon.
There are two basic types of ankle replacements: unconstrained meniscal bearing ankle prostheses and semi-constrained fixed bearing ankle prostheses. An example of a semi-constrained fixed bearing ankle prosthesis is the Agility Ankle by DePuy. Semi-constrained fixed bearing ankle prostheses include one articulating interface, i.e., the interface between the concave polymer bearing surface and the convex metal bearing surface. In semi-constrained prostheses, the concave polymer bearing is locked in place relative to the tibia. Further, the convex metal bearing surface is locked in place relative to the talus. Thus, semi-constrained fixed bearing ankle prostheses provide the patient with only a limited amount of ankle movement.
Unconstrained meniscal bearing ankle prostheses, in contrast, include two articulating interfaces, allowing additional degrees of freedom. One articulating interface is between a tibial component and a meniscal bearing. The other articulating interface is between the meniscal bearing and a talar component. Examples of unconstrained meniscal bearing ankle prostheses are the S.T.A.R. prosthesis from Link, and the B-P ankle from Endotech, both of which are loaded from the front of the ankle. There are several problems with such unconstrained ankle prostheses, a few of which will be listed here as examples. First, ankle stability is poor. Specifically, the meniscal polymer bearing can become dislocated. Second, an unconstrained ankle prosthesis cannot be used on patients with excessive varus/valgus misalignment or a soft tissue weakness that affects the alignment or stability of the foot. Third, fibrous tissues tend to encapsulate the bearing, thereby limiting motion. Fourth, the tibial component is known to migrate due to the small amount of bone contact area.
Consequently, there is a need for an ankle prosthesis system that allows greater freedom of movement than does a semi-constrained fixed bearing ankle prosthesis and that does not have the disadvantages of an unconstrained meniscal bearing ankle prosthesis.